1. Reference to Related Applications
This application claims the priority of Japan application No. 63,270748 filed Oct. 28, 1988, which is incorporated herein by reference.
2. Field of the Invention
This invention relates to a method and a node circuit of a packet switching system for the routing of packets, and relates particularly to a method and a node circuit for routing a bursty data transmission.
3. Description of the Related Art
An example of a packet communication system which has been previously proposed to handle bursty data in multimedia information communications is described in "Statistical Switching Architecture for Future Services" ISS' 84 (May 1984). This system features a simple communication protocol and a self-routing method.
However, the previously proposed system is merely a kind of routing apparatus which depends for its routing instructions only on the routing information (address) included in the packets.
Therefore, the system does not take into account the conditions of the transmission links and does not transmit or interpret information concerning the packets themselves. Accordingly, it cannot prevent loss of packets caused by an overflow of switching buffers.
Generally, when such bursty data is transmitted in multimedia information communications, the buffer will overflow and packets will be lost, resulting in a reduction in the quality of communication, because bursty data transmissions have the specific characteristics described below:
Generally, the terms "bursty data transmission" and "bursty messages" mean a series of alternating continuous transmissions of significant data separated by pauses or non-significant data transmissions.
For example, when transmitting a message in the form of a moving image (i.e. video transmission), first an initial transmission of data for an entire image (a continuous data transmission) will be sent, and after that, a stream of short bursts of data representing successive changed portions of the image, which are overwritten on the initial entire image, will be sent to realize an efficient data transmission.
In a multimedia communication system capable, for example, of a voice data transmission or an image data transmission or both, especially in a case of transmitting moving images, such bursty data transmission is often used to reduce the total amount of data transmission by eliminating redundancy in the transmitted data.
FIGS. 1(a), 1(b) and 1(c) illustrate a bursty data transmission and two different forms of corresponding packets used to transmit the produced data. As shown in FIG. 1(a), first a long block of data high in information content is produced, and then, a stream of shorter, less dense blocks of data with pauses interposed therebetween is produced. If the data is transmitted in a variable length packet form, special flag packets representing no data transmission are transmitted during the pauses between data packets, as shown in FIG. 1(b). If the data is transmitted in a fixed length packet form, an empty packet which has fixed length, but with no significant data content, is transmitted during each pause.
Generally, the transmission characteristics of the required transmission path are determined by the required maximum transmission speed, and in many cases this depends on the speed needed to transmit the above-mentioned initial continuous data. If the transmission path is designed merely to carry data at an estimated average transmission speed, the buffer may overflow during the initial continuous data transmission if the actual average transmission speed is greater than the estimated average transmission speed.
On the other hand, since there are many pauses during data transmission of a bursty message, if the transmission path is merely designed to carry data at the maximum transmission speed so as to transmit the initial continuous data, the efficiency of transmission will be wasteful and the system will be more expensive than necessary.